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Whenever you see open water (either running or still), you can often feel a little closer to your primordial roots — after all, life (and that includes you) came from the oceans. And so, whenever you see a foam sitting on top of that water, you automatically recoil and think of some kind of pollution.

In really big seas, the foam can be a few metres thick on the ocean and lining the beaches. But in truth, most foam comes from natural causes, whereas most chemical pollution is invisible.

There's an old saying in physics which runs: "The surface is the devil", and it means that surfaces or boundaries are difficult to understand. In marine and freshwater ecosystems, the boundary between the air and the water is called the surface microlayer. All organic and inorganic materials that travel between the atmosphere above and the water below have to travel through this surface microlayer. Indeed, substances, chemicals and particles will tend to accumulate (at least for a little while) in this surface microlayer — and this is where foams will form.

A foam is defined as a gas being spread throughout a liquid, and separated by very thin liquid films — in other words, a foam is lots of bubbles stuck together.

The trouble is that foams are "thermodynamically unstable", or in plain English, they tend to collapse. If you get some pure water and try to whip up some bubbles with a kitchen whisk or a fork, you'll see that the bubbles will collapse in much less than a second. But after a big storm, that metre-or-more of foam will sit on the beach for hours, or days. How come?

It's the natural surface tension of water that's the problem.

Water molecules are H2O, and are made of two atoms of hydrogen (that's the H) and a single atom of oxygen (that's the O). They look like a tiny boomerang, with the oxygen atom in the middle where the two arms of the boomerang meet, and the hydrogen atoms on the ends of the arms. By the way, the angle that the two arms meet at is a bit bigger than a right angle.

Now this boomerang carries some electrical charges on itself. The two hydrogen atoms on the ends are positively charged, while the single oxygen atom is negatively charged. Now we know that positive and negative charges will attract, so water molecules have a tendency to stick to each other.

Think about one single water molecule sitting just on the surface of the water. There are water molecules to its left and right, and in front of it and behind it, and below it — but no water molecules above it.

Let's look at our single boomerang of a water molecule. It will be attracted to the water molecule just off to the left of it. But that attractive force will be balanced by the pull of a different water molecule just off to the right. So our single water molecule in the middle will be attracted equally, and will tend to go neither left nor right. In the same way, the forces from the water molecules in front of it and behind it will also balance out — so it will tend to go neither backward nor forward.

But the forces situation is unbalanced with regard to up and down. Yes, there is another water molecule directly below it, pulling it inwards into the bulk of the water. But there is no water molecule above it, only air.

So there is a net inward force on all the water molecules at the surface. We call this force surface tension. That's why water comes out of a dripping tap all neatly rolled up into a little ball — surface tension.

Think about the water molecules that are sitting inside the incredibly thin wall of a bubble. The surface tension force is pulling those water molecules back into the bulk of the water — and as it does, the bubble collapses.

To make the bubbles in sea foam stable and last for a long time, you have to stop the water draining out of the walls. You can do by reducing the surface tension, or by making the bulk liquid more viscous, or by adding particles to it.

So how does nature, and the human race, do all this to bodies of water? Well, I'll talk more about that next time...